Idanthyrsus willora, Hutchings & Capa & Peart, 2012

Hutchings, Pat, Capa, María & Peart, Rachael, 2012, 3306, Zootaxa 3306, pp. 1-60 : 18-21

Idanthyrsus willora n. sp.

Figures 1E–F, 6A, 9, 10, Table 1

Material examined. HOLOTYPE: 1, WAM V7852 *, West Australia: Quondong, 17°34'S 122°09'E, 01.vii.1975, collected beneath intertidal rock slabs, 45 mm in length, 5 mm in width, 48 chaetigers . PARATYPES: 1, WAM V7853 *, 32 mm in length, 5 mm in width, 47 chaetigers, 1, AM W37779 (mounted for SEM on two pins), both paratypes from same sample as holotype .

Additional material examined. Western Australia: Dampier Archipelago Kendrew Isl. 20°28'30"S 116°325'12"E 7.v. 1973, 8 m, 1, WAM V3812 ; transect N, Kendrew Isl. 20°28'28"S 116°25'12"E 11.ii.1973, 1– 2 m, 1 WAM V899 ; Lady Nova Flat, NE Rosemary Isl. 2.xi.1971, intertidal on oyster, 3, WAM V677 ; northeast side of Cape Naturaliste, 33°32'S 115°00"E, 26.xii.1958, low tide, 2, WAM V855 , under stones; Port Denison, 32°10'51"S 115°45'12"E, 27.x.1973, 9.7–10.6 m, 1, WAM V626 , on pieces of reef; Near Little Isl. off Sorento, 31°34'36"S 115°44.22"E, vi. 1974, 3 m on bivalve shells, 1, WAM V3820 ; Cottesloe Reef, 32°35'39"S 115°58.29"E, 14.vi.1973, intertidal on Ninella mollusc, 3, WAM V313 . Northern Territory: Trepang Bay , 11°13'S 131°57'E, 15.x. 1981, 5 m, 1, NTM W26 *, from edge of reef .

Description. Holotype robust, compact ( Fig. 1E), with dark pigment patches throughout ventrum, on abdominal tori and caudas. Operculum partially fused lobes ( Figs 9A, B, 10F), with an outer row of 17 pairs of paleae, with cylindrical and smooth shafts, flat and curved blades, with sharp-tipped denticles present on anterior half of shaft ( Fig. 10C), length of which declines towards to tip, more splayed basally ( Figs 1F, 9A, 10C). Inner row with 13 pairs of cylindrical paleae, slender, smooth, tapering gently with slightly curved tips ( Fig. 10C, D). Opercular papillae, 12 pairs, inserted externally to outer paleae on each lobe ( Fig. 1E). Four pairs of nuchal spines (hooks) on each side ( Figs 1F, 9A), long shafted, and cylindrical, with subacute tips, lacking limbation; longest ones with more rounded tips, weakly recurved ( Figs 1E, F, 9D, 10B). Tentacular filaments compound, arranged in 20 transverse, long rows ( Fig. 9A, B). Small median organ present at dorsal juncture of opercular lobes ( Figs 1F, 9A, 10A). Eyespots along sides of median ridge. Palps tapered, about one third length of opercular lobes. Segment 1 (chaetiger 1) with lobe-shaped neuropodia ( Fig. 9C), with thin and flattened capillary neurochaetae, with serrations on their margins, decreasing in size towards fine tips ( Fig. 10E). Segment 2 (chaetiger 2) with four pairs of large triangular-shaped lobes, connecting branchiae to neuropodia ( Figs 1F, 9B, C). Neuropodia of chaetiger 1 and 2 not vertically aligned, those of chaetiger 2 situated more laterally ( Fig. 9C), neurochaetae similar in structure. Fourty-five pairs of dorsal branchiae present from chaetiger 2, wide based ( Fig. 9C), not meeting mid dorsally, continuing along entire length of body, but diminishing in size posteriorly so by posterior segments very small ( Figs 1E, 9E). Segments 3–5 (parathoracic) with two types of notochaetae arranged in two rows, 11 lanceolate slightly curved forming a shallow scoop with slightly twisted frayed tips and blades and 11 short, thinner lanceolate chaetae inserted between each of larger ones ( Fig. 10F). Segments 3–5, with lanceolate neurochaetae arranged in two tiers and significantly different in width, largest ones with smooth blades and smaller ones with textured blades. Parathoracic notochaetae more robust than parathoracic neurochaetae. Abdominal region with 43 chaetigers. Notopodia as transverse tori, with uncini, numbers per torus decreasing posteriorly. Each uncinus with two rows of vertical teeth, each with eight teeth ( Fig. 10H). Neuropodia with capillaries becoming longer posteriorly, with thin, flattened blades ornamented with thecal laminar extensions, distally forming oblique rows with elongate thin tips ( Fig. 10G), similar in structure on all abdominal chaetigers. Cauda smooth about one third the length of abdomen ( Figs 1E, 9E).

Variation. Material examined, including sexually mature females, varies from 10–35 mm in length without cauda, 2–6 mm in width, with 36–48 chaetigers, 14–18 pairs of outer paleae, 8–15 pairs of inner paleae, 8–17 pairs of opercular papillae, 1–4 pairs of nuchal hooks, 12–20 rows of compound tentacular filaments and 29–45 pairs of dorsal branchiae. These variations can be mainly attributed to size of the specimens, with larger animals tending to have more paleae, papillae, tentacular filaments and dorsal branchiae than smaller ones.

Remarks. Idanthyrsus willora is described as a new species because of the presence of four pairs of lateral lobes on segment two, which have irregular margins. The number of lateral lobes is a feature only shared with I. saxicavus ( Baird, 1863) although in the latter species these lobes are long and digitiform ( Kirtley 1994: Fig. 6.15.1). These two species are also distinguished by the shape of outer paleae increasing in size slightly towards tips in the new species, while in I. saxicavus they are similar in size all along the paleae. Idanthyrsus willora n. sp., can be distinguished from the other Australian species by several features. Idanthyrsus willora has straight denticles increasing in length towards the tip on the outer paleae while there are curved denticles similar in length along the blade in I. australiensis . The number and shape of lateral lobes on segment 2 also varies, (four large triangular ones in I. willora n. sp., two poorly developed in I. nesos n. sp., and three triangular ones in I. australiensis ). Idanthyrsus willora n. sp., also differs from I. nesos n. sp., in having branchiae continuing to the far posterior segments. Idanthyrsus willora n. sp., can be separated from other species recorded from the Indo-Pacific by the absence of branchiae on posterior segments, whereas they are present in L. boninensis Nishi & Kirtley, 1999 , described from Japan. Another Japanese species, I. okudai Kirtley, 1994 , can be separated from I. willora n. sp., based on the number of pairs of lobes on segment 2 (four in I. willora n. sp., and only two in I. okudai ). Two other species from from Sri Lanka, I. kornickeri Kirtley, 1994 and I. bicornis ( Schmarda 1861) , both poorly described (see Table 1), are distinguished from I. willora n. sp., in the shape and arrangement of the denticles on the outer opercular paleae which are curved and similar in length along the blade in I. kornickeri , in contrast to those from I. willora n. sp., which are straight and increase in length towards the tip. Idanthyrsus bicornis has these denticles arranged opposite to each other along the shaft, and in I. willora they are arranged in an alternate pattern on both margins. More over, the nuchal hooks in I. bicornis are limbate whereas they are not in I. willora n.sp.

Distribution. Western Australia and Northern Territory.

Habitat. Specimens found in the intertidal but there is no information available as to whether this species is gregarious or occurs as single individuals, since specimens examined had all been removed from their tubes. Some of the records indicate they were attached to mollusc shells.

Etymology. The specific name willora is the Aboriginal word for the type locality Quondong ( Endacott 1973).